Vertical scanning of an LCD (Liquid Crystal Display) relates to providing image data to an LCD pixel array. Vertical scan rate refers to the number of times, per unit time, that an LCD pixel array is refreshed (i.e., redrawn). Vertical scanning can be implemented with a shift register-based scanner or an addressable scanner.
A shift register-base scanner selects each row sequentially, from top-to-bottom or from bottom-to-top. The order of the row selection does not change—only the direction of the sequential section may change.
An addressable scanner provides more flexibility with image construction, since each row can be selected independently. This feature allows the LCD to do a “line copying” function, in which the video data on one row can be “copied” to other rows in a short period of time. This is useful when the same data is to be written, within tight timing constraints, to many rows.
The copying function relies on storing the video voltage on the column capacitance, and turning on the copied rows one-by-one. Due to leakage that may be associated with the column capacitance, the stored video voltage may change over the time it takes to write to multiple rows. Such a voltage change may result in the rows that are copied later appearing lighter than the rows that are copied earlier.